The invention relates to detection of contaminants in containers, particularly contaminants whose presence is manifested by turbidity of a liquid or foam on top of the liquid.
The popularity of refillable containers has increased as the costs, both social and financial, associated with disposal of packaging have become less acceptable. For example, in many countries, water and other beverages are sold in refillable bottles. These bottles are often made from a type of plastic known as polyethylene terephthalate.
After use, refillable containers are returned to a bottling plant where they are cleaned and inspected before being refilled. This inspection, in addition to checking for physical damage such as cracks, screens the containers to eliminate those that include contaminants that might degrade the flavor, safety, or other qualities of the product that they contain. The risk of contamination is greater when a container is made from plastic, as opposed to glass, because some contaminants can be absorbed into the plastic walls of the container. Absorbed contaminants can persist despite cleaning procedures, and can later leach into the product.
Though some contaminants, such as detergents and fabric softeners, are visibly colored and can be detected by human inspectors, such human visual inspection is undesirable when bottles or other containers are moving on high speed conveyors and stopping or touching the bottles to perform an inspection is undesirable or overly expensive. Moreover, such human visual inspection is subject to lapses in attention by the inspectors.
As an alternative, it has been suggested to use spectrophotometric instrumentation to automatically detect colored contaminants. Spectrophotometric instrumentation for color detection is well known in many fields, including laboratory analysis of chemical solutions, and quality control functions in the paint, fabric, and photographic industries. In general, spectrophotometric analysis of liquid samples is based on Beer's Law, which states that the optical density (i.e., the log ratio of transmitted or detected light intensity to incident light intensity) is directly proportional to the concentration of the chemical compound giving rise to the absorption of light. Beer's law is discussed, for example, in H. A. Strobel, Chemical Instrumentation, pp. 148-53 (1960, Addison Wesley, Reading, Mass.). Beer's law is limited in that it can only be applied if all of the detected light travels the same distance through the absorbing medium. In chemical spectrophotometric analysis, this is done by placing the liquid sample in a cuvette, or optical cell, having parallel windows that are typically spaced apart by ten millimeters.
When a container is mostly filled with liquid, a narrow optical beam can be directed radially through the container so that it intersects the major vertical axis of the container in a region where the wall of the container is substantially parallel to the major vertical axis. In the case of a cylindrical container such as a bottle, if the beam is very narrow relative to diameter of the bottle, all of the detected light travels over substantially the same path length through the bottle, and the geometrical conditions for Beer's law are satisfied.
However, when a bottle or other container only contains a few millimeters of residual liquid, it becomes much more difficult to utilize Beer's law. For example, in refillable plastic beverage bottles, the walls of the bottle curve inward near the bottom of the bottle and are not parallel to the major axis of the bottle. Also, many refillable plastic bottles include a dome in the bottom of the bottle. In these situations, refraction and reflection result in the detected light travelling over a variety of path lengths, and Beer's law cannot be applied.
One way of dealing with these difficulties is to mechanically tilt the bottle to pool the residual liquid in a corner of the bottle and arrange the incident beam of light perpendicular to the face of the bottle to minimize refractive effects. In this case, as long as the beam of light is sufficiently thin and the dome avoided, the path length will be well defined and free of multiple reflections, and conventional Beer's law analysis can be applied. However, mechanical arrangements to provide such tilting may be complex and difficult to integrate with a high speed conveyor utilized in the processing of refillable containers.